Various systems of this type frequently coexist over contiguous and/or partially overlapping but not totally overlapping geographical areas. For example, in a country having a federal type administrative organization, as is the case in the United States of America, the decisions regarding the deployment of radiocommunications systems for the police, for example, can be taken in an unconcerted and/or uncoordinated manner in bordering states. Nevertheless, the users, in this instance the police agencies of the states, must be able to carry out joint operations such as for example the pursuit of a villain fleeing from one state to another. For this reason, interoperability between various systems is necessary.
The invention aims to propose a solution to allow the interoperability between two mobile radio systems having mutually incompatible respective radio interfaces.
Each system comprises on the one hand mobile terminals such as portable terminals (installed on board a vehicle) and/or terminals which can be carried (by a user) and on the other hand a fixed infrastructure (network) comprising base stations distributed over the geographical area covered by the system and equipment for interconnection between the base stations and the fixed infrastructure of another system. The radio interface of a mobile radio system designates the set of protocols and of specifications which govern the exchange of radio signals between the base stations and the mobile terminals of the system. The characteristics of the radio interface are given, in particular, by the frequency band used, the width of the spectrum of the radio signals, the type of modulation, the type of coding of the information, the frame structure, etc.
Conventionally, a call originating from a mobile terminal intended for another terminal, mobile or fixed, is referred to as an outgoing call. The term “outgoing” is therefore to be regarded from the point of view of the mobile terminal from which the call originates.
In what follows, the case illustrated in FIG. 1 of a first and of a second mobile radio system is considered. The first system comprises on the one hand a network R1 having at least one base station BTS1, and on the other hand at least one mobile terminal TR1. The base station BTS1 and the mobile terminal TR1 can exchange radio signals via a radio interface I1. The second mobile radio system comprises on the one hand a network R2 having at least one base station BTS2, and on the other hand at least one mobile terminal TR2. The base station BTS2 and the mobile terminal TR2 can exchange radio signals over a radio interface I2. The base stations BTS1 and BTS2 of the networks R1 and R2 respectively are linked in a wire manner to an inter-system interface ISI carrying out the interconnection of the networks R1 and R2. Such an interface will be able to carry out a trans-encryption and/or trans-vocoding operation when the modes of encryption and/or of vocoding of the first and second systems are mutually incompatible.
In one example, the second system is regarded as constituting a new version or new generation, of the first system. In this example, the first system is moreover regarded as being the Project 25 system of the APCO (standing for “Association of Public-Safety Communications Officials-International, Inc.”). The second system therefore consists, in this example, of a new generation of the Project 25 system. This is why, in what follows, the first system will sometimes be referred to as Project 25—Phase I or old generation system, whereas the second radiocommunications system will sometimes be referred to as Project 25—Phase II or new generation system.
In order to improve the spectral efficiency, the Project 25—Phase II system uses channels whose equivalent bandwidth is equal to 6.25 kHz per voice communication, as replacement for the channels of the Project 25—Phase I system whose equivalent bandwidth is equal to 12.5 kHz per voice communication. This system may for example be a TDMA system of order 2, the physical channels with a width of 12.5 kHz being able to carry two simultaneous voice communications so as to obtain a total efficiency of 6.25 kHz per communication. For this reason, and for others, the characteristics of the radio interface I2 of the Project 25—Phase II system are different from the radio interface I1 of the Project 25—Phase I system. It follows from this that the radio interface I2 is incompatible with the radio interface I1.
In the configuration illustrated in FIG. 1, the mobile terminal TR1 is in the area of coverage of the network R1, and more particularly of the base station BTS1 of the old generation system. Moreover, the mobile terminal TR2 is in the area of coverage of the network R2 and more particularly of the base station BTS2 of the new generation system. When the mobile terminal TR1 wishes to enter into communication with the mobile terminal TR2, it sets up a communication with the base station BTS1 according to the protocol of the radio interface I1. The call is then processed by the network R1 so as to be routed to the inter-system interface ISI, since the network R1 recognizes a call intended for a terminal belonging to the new generation system, owing, for example, to the fact that the call numbers identifying the terminals of this system possess a characteristic prefix. The network R2 of the new generation system receives an incoming call on the inter-system interface ISI intended for the mobile terminal TR2. It sets up this call, via the base station BTS2, according to the protocol of the radio interface I2 of the new generation system. Call set-up is thus carried out simply. Specifically each mobile terminal, in particular the calling mobile terminal TR1, is in the area of coverage of the network of the system to which it belongs. The exchange of voice data can then take place between the terminal TR1 and the terminal TR2, possibly with an operation of trans-encryption and/or of trans-vocoding in the inter-system interface ISI.
Nevertheless it may happen that a mobile terminal exits the area of coverage of the network of the system to which it belongs.
Thus, in the configuration illustrated in FIG. 2, the terminal TR2 of the new generation system S2 has left the area of coverage of the network R2 of the new generation system and is in the area of coverage of the network R1 of the old generation system, and more particularly of the base station BTS1 of this system. This configuration still does not raise any problem in respect of the establishment of a call originating from the terminal TR2 intended for the terminal TR1. Indeed, the terminal TR2 of the new generation system is in general capable of using the radio interface I1 of the old generation system. In fact it comprises means for emulating this interface. Interoperability between the two systems is then obtained in a simple and efficient manner.
Nevertheless, a problem arises in the reverse configuration, illustrated in FIG. 3, according to which a terminal TR1 of the old generation system has left the area of coverage of the network R1 of this system and is in the area of coverage of the network R2 of the new generation system, and more particularly of the base station BTS2 of this system. Specifically, the terminal TR1 being of an old generation, it cannot use the radio interface I2 of the new generation system.
One solution would consist in permanently dedicating, in the new generation system, resources necessary for the emulation of the radio interface I1 of the old generation system. In the case of an FDMA (standing for “Frequency Division Multiple Access”) type system, such as the Project 25 system, such resources would consist of a conventional physical channel set up permanently on one of the frequencies specific of each base station of the new generation system, such as the base station BTS2. This solution is nevertheless inconceivable since it excessively penalizes the spectral efficiency of the new generation system. In particular, for base stations installed in a rural area, and for which in principle just two traffic physical channels are sufficient, this solution would represent a 50% overhead in terms of resources dedicated to interoperability. These resources being necessary on average in only 1% of cases, this overhead is prohibitive.